In respiratory epithelia, goblet cells secrete mucus to trap foreign particles and invading pathogens; multiciliated cells (MCCs) provide synchronized beating of motile cilia, driving the extruded mucus out of the respiratory tract. The coordinated functions of MCCs and goblet cells constitute the basis for defense mechanism against respiratory infections. A single MCC contains hundreds of motile cilia that beat coordinately to generate continuous and directional movement of extracellular fluid for pulmonary clearance. Hence the motile ciliogenesis in MCCs is particularly important for the pulmonary defense in respiratory epithelia. Although it has become increasingly clear that non-coding RNAs are integral components of the molecular network for development and disease, most studies on motile ciliogenesis and MCC bioology have focused on protein- coding genes. Using mouse models, our preliminary studies identified miR-200 miRNAs with an essential role in respiratory epithelia. The miR-200 family consists of five highly homologous and evolutionarily conserved miRNAs that collectively exhibit a high-level expression in respiratory epithelia, and particularly, in multiciliated cells. The redundancy of the miR-200 family in the mammalian genome, combined with their strong expression patterns in multiciliated cells, confer a robust functional regulation on motile ciliogenesis. Mice deficient for all miR-200 miRNAs die postnatally, exhibiting strong respiratory dysfunction, excessive mucus accumulation and impaired motile ciliogenesis. Using mouse and frog genetics, cell biology and molecular biology approaches, we proposed to carefully characterize the phenotype in miR-200 deficient MCCs in mouse and in human, with a particular focus on motile ciliagenesis. In addition, we propose to investigate the molecular mechanisms underlying the miR-200 functions during motile ciliogenesis. Taken together, these proposed studies will not only deepen our understanding on the molecular basis of motile ciliogenesis, but also provide important insights into the development of new diagnostic markers and therapeutical agents for treating respiratory conditions.